1. Field of the Invention
The present invention relates generally to semiconductor packaging. More particularly, the present invention relates to wafer level multichip packaging such as, for example, a system in a package solution.
2. State of the Art
Semiconductor dice (also referred to as chips/dice herein) are found in many electronic products today. As semiconductor dice get smaller and more complex, the problem of making electrical connections between semiconductor dice, connections to carrier substrates such as printed circuit boards, and connections to intermediate substrates such as multichip modules which are, in turn, connected to carrier substrates, has been addressed with a variety of constantly evolving solutions.
One of the earlier solutions included wire bonding from signal connection devices, such as bond pads of a semiconductor die, to pins or leads of a lead frame contained in a ceramic or plastic package. Finished packages are mounted to a carrier substrate, such as a printed circuit board, where the pins or leads make electrical connection with contact pads on the carrier substrate.
The term “signal connection devices” as used herein regarding semiconductor devices includes not only contact pads of a substrate and bond pads of a semiconductor device, but also I/O connections for a semiconductor device created by adding circuitry from bond pads located on the active surface of the semiconductor device to different locations on the active surface of the semiconductor device. Such additional circuitry is typically effected using a so-called “redistribution layer” extending over the active surface or a surface of a semiconductor die.
An evolution of electrical connection technology occurred when multiple semiconductor dice were mounted on an intermediate substrate. In this instance, the semiconductor dice are typically connected to a lead frame by way of bonding wires. Signals, or electrical connections, required for coupling with an external device, such as a circuit board, are brought out to contact pads, pins or leads of the multichip module package. Other signals or electrical interconnections may be established between multiple semiconductor dice by way of circuitry formed on the intermediate substrate.
In these solutions, using wires for connecting a semiconductor die to a substrate and wire bonding processes can create problems. Such problems may include, for example, size and pitch (spacing) requirements for the bond pads of the semiconductor die and contact pads of the substrate; inductance in the signals due to the long curved wires; wire bond breakage and wire sweep causing shorting between adjacent wires; and high signal frequency semiconductor dice making the wire bonding process difficult and expensive.
Flip-chip technologies using solder balls or bumps have helped to alleviate some of these problems. For example, instead of wire bonding, conductive bumps such as, for example, balls of solder may be formed at the locations of the bond pads of a semiconductor die. A specialized lead frame, a dielectric tape carrying circuit traces as used in tape automated bonding processes, or other carrier substrates such as a printed wiring board may have electrical connection locations such as terminals which correspond to the placement of the solder balls on the bond pads of the semiconductor die. The semiconductor die is “flipped” upside down so the solder balls are placed, for example, on the contact pads of a carrier substrate. A solder reflow process heats the solder balls until the solder begins to flow and bond with a corresponding contact pad of a carrier substrate. Upon cooling, the solder forms both mechanical and electrical connections between the carrier substrate and the semiconductor die. This packaging solution may alleviate at least some of the inductance problems, allowing for higher frequency performance and better signal integrity of the semiconductor die. Also, to a certain extent, it allows the contact pads of a substrate where bumps were formed to be larger, more widely pitched and placed anywhere on the semiconductor die active surface rather than just around the periphery or down the center thereof.
Chip scale packaging has evolved from various standard flip-chip processes to a configuration wherein the size of a package is reduced to only slightly larger than the size of the semiconductor die. Chip scale packages are typically created using an interposer substrate. The semiconductor die, with solder balls or bumps such as described above, is attached and electrically connected to the interposer substrate and an encapsulation material is applied over the chip for protection thereof from the elements. The interposer substrate can redistribute signal connections to new locations so they are physically positioned in a desired pattern or arrangement, or to just a different pitch more suitable for mounting to an interposer substrate. An additional set of conductive bumps may then be formed at other contact pad locations on the interposer substrate. The resulting package may then be attached to a carrier substrate such as a printed circuit board.
Chip scale packaging enables small packages using desired ball grid arrays or fine ball grid arrays. However, the interposer substrate is typically made of an organic material which is the same as, or similar to, that used for printed circuit boards. There is conventionally a significant mismatch in the coefficients of thermal expansion (CTE) of the interposer substrate and the semiconductor die, often resulting in substantial stress on the mechanical and electrical interconnections formed between the semiconductor die and interposer substrate (e.g., a reflowed solder connection) during the normal thermal cycling during normal operation of the semiconductor die. The use of a ceramic substrate may alleviate some of the CTE mismatch concerns but at a considerably higher cost relative to more conventional interposer substrates.
Another advance in the area of multichip modules includes wafer scale integration. Wafer scale integration generally comprises fabricating multiple types of functional dice on a single wafer. For example, a four-chip system may be created by placing a microprocessor next to a memory controller and two memory-type semiconductor die. This pattern may then be repeated across the entire wafer. After fabrication, the wafer is sawed into individual segments with each segment containing the four different functions. However, this approach has not been a very satisfactory solution due to yield problems created by the variations in processes for forming processors and various types of memory-type semiconductor dice. For example, if a defect causes any one of the four functions to be inoperable, the entire segment is defective and not usable.
In addition to that described above, there have been advances in bump technologies where the conductive bumps act as the signal connection device. Conventional solder bumps, in some cases, have been replaced by stud bumps. Stud bumps have conventionally been gold, but copper and plated-type stud bumps have also been used recently. The stud bumps may actually comprise short wires or wire stubs applied to a semiconductor die using a conventional wire bonding process. Stud bumping has the advantages of using a more cost effective wire bonding process for application of the bumps in comparison to the more complex, multistep solder bumping process. Further, conductive and conductor-filled adhesives have also been employed to attach the conductive bumps to a carrier substrate. The conductive or conductor-filled adhesive may provide an amount of flexibility to the mechanical and electrical connection, thereby compensating for some of the problems associated with the mismatch of CTE often associated with solder bump processes as discussed above.
However, in light of the advances made in fabricating semiconductor device packages, there is a continued need for a reliable, cost effective solution with a higher integration of various functional dice in a single package to produce, for example, a system on a chip solution. There is also a need to create smaller packages with more consistent thermal expansion properties while enabling the redistribution of signal connection devices of the various dice to a more convenient, possibly denser, and optionally standard configuration for attachment to a carrier substrate, the redistribution layers also enabling higher frequency signal performance.
Finally, it would be advantageous to provide a system on a chip packaging solution using known good dice, such use thereby increasing the yield of usable packages and, thus, improving the efficiency and cost effectiveness associated with producing such packages.